1. Field of the Invention
The present invention relates generally to cleaning units, process cartridges, and image-forming apparatuses, and more particularly to a cleaning unit employed in electrophotographic or electrostatographic image-forming apparatuses such as copiers, printers, and facsimile machines, a process cartridge including the cleaning unit, and an image-forming apparatus including the process cartridge.
2. Description of the Related Art
According to electrophotographic image-forming apparatuses such as printers, facsimile machines, and copiers, an electrostatic latent image formed by charging the surface of an image carrier and exposing the surface to light is developed with colored toner so that a toner image as a visible image is formed. Then, the toner image is, directly or after being transferred to an intermediate transfer member, transferred to a transfer medium such as transfer paper, and is fixed by a heating roller so that an image is formed.
Generally, untransferred toner remains on the surface of the image carrier after the transfer of the toner image. Accordingly, it is necessary to remove the remaining (residual) toner by cleaning means prior to the next image-forming process. Generally, the cleaning means removes not only the remaining toner but also other foreign substances adhering to the surface of the image carrier.
As the cleaning means for removing the remaining toner after the transfer of the toner image, a variety of methods such as those using a fur brush, a magnetic brush, and a cleaning blade whose material is an elastic body such as rubber have been employed. The method of scraping off the remaining toner by rubbing the image carrier with the cleaning blade (a blade cleaning method) is commonly employed since the method is inexpensive and highly stable in performance. The elastic body employed as the material of the cleaning blade is often polyurethane rubber having good wear resistance.
In recent years, toner particles have become smaller in size (diameter) and spherical in shape in order to achieve high image quality in full-color image-forming apparatuses. The dot reproducibility of a toner image formed on the surface of the image carrier can be improved by reducing the particle size of toner, and the developability and the transferability of the toner can be improved by making the particle shape spherical.
As a method of forming spherical toner (particles) for achieving high image quality, the conventional pulverization method has been replaced by a polymerizing method accompanied by the chemical reaction of polymerization. The polymerizing method, which has variations within the same category of polymerization reaction, is advantageous in that a pulverization and classification process employed in forming pulverized toner can be dispensed with or greatly simplified.
The use of the above-described toner of small-size spherical particles, which hereinafter may be referred to as spherical toner, may cause a problem in that good cleaning performance cannot be obtained by the blade cleaning method. That is, it is difficult to completely remove the remaining toner on the surface of the image carrier with the cleaning means, thus resulting in poor cleaning performance or imperfect cleaning.
It is known that this poor cleaning performance or imperfect cleaning occurs even in the case of making the conventional pulverized toner small in particle size and spherical in particle shape by mechanical processing (re-pulverization) or heat treatment. When toner becomes small in particle size and spherical in particle shape, the blade cleaning method cannot achieve good cleaning performance irrespective of the method of manufacturing the toner.
The toner that has been cleaned imperfectly results in a quality defect of the next image to be formed and output. Particularly, if a charger is of a contact type having a roller shape, the toner that cannot be removed (cleaned) by the cleaning blade may be deposited on the roller-like charger to cause incomplete charging, thus exerting a great influence.
Particularly, a cleaning characteristic remarkably worsens when the (particle) circularity of toner, which is described in detail below, is close to one, that is, when almost all of the toner particles are spheres in shape. Further, even if the toner has a circularity less than or equal to 0.95 in shape, the toner includes substantially spherical particles since the toner particles have a shape distribution. Accordingly, the cleaning characteristic tends to worsen over time.
Further, the cleaning characteristic tends to worsen as the particles of toner employed for development become smaller in size. The image-forming apparatuses are employed within the temperature range of approximately 10 to 30° C. Particularly, at low temperatures, the cleaning characteristic shows remarkable deterioration.
According to the cleaning method using the cleaning blade, the remaining toner is scraped off by rubbing the surface of the image carrier with the rubber blade as described above. Therefore, the tip of the edge of the rubber blade deforms due to the frictional resistance between the image carrier and the rubber blade, thus forming a minute wedge-like space therebetween. The smaller the toner particles are, the more easily the toner particles enter the tip of the edge in this space. The toner particles that have entered the tip of the edge are difficult to replace, thus forming a non-fluid area.
Further, it is easier to perform closest packing on the spherical toner than on irregularly shaped toner. Therefore, the spherical toner is likely to be compacted in the minute space in the vicinity of the contact point between the edge of the cleaning blade and the image carrier. When the frictional resistance between the toner in the non-fluid area and the image carrier is relatively small so that the toner slides relative to the image carrier, imperfect cleaning is prevented from occurring. However, when an external additive is removed from the toner by its sliding and rubbing on the image carrier so that the friction between the toner and the image carrier increases, the spherical toner, whose rolling friction is smaller than that of the conventional irregularly shaped toner, starts to roll and slips through between the cleaning blade and the image carrier.
Japanese Laid-Open Patent Application No. 2001-188452 (Prior Art 1) discloses a cleaning unit that, in order to efficiently remove residual toner on the image carrier of an image-forming apparatus using spherical toner manufactured by the polymerizing method, includes: a cleaning blade scraping the residual toner off the surface of the photosensitive body (image carrier) after the transfer of a toner image; and a cleaning brush disposed on the upstream side of the cleaning blade in a direction in which the photosensitive body moves, the cleaning brush pulverizing the residual toner so as to produce pulverized toner on the photosensitive body.
Japanese Laid-Open Patent Application No. 2000-267536 (Prior Art 2) discloses an image-forming apparatus that, in order to improve the cleaning characteristic of a cleaning blade for cleaning an image carrier with respect to spherical toner, includes: a toner image carrier whose surface carrying a toner image formed by spherical toner rotates through a transfer area and a cleaning area; a transfer device that transfers the toner image on the surface of the toner image carrier onto a transfer material when the surface passes through the transfer area; a cleaning blade formed of an elastic member, the cleaning blade having a blade edge that comes into frictional contact with the surface of the toner image carrier so as to remove residual toner thereon when the surface passes through the cleaning area; and a toner image carrier cleaner having a powdery mixture material applied to the blade edge of the cleaning blade, the powdery mixture material being composed of a powdery lubricant and irregularly shaped toner whose average particle size is smaller than that of the spherical toner.
Japanese Laid-Open Patent Application No. S62-201489 (Prior Art 3) discloses a cleaning unit for an image-forming apparatus, the cleaning unit having a cleaning blade that is forced to vibrate in order to shake off toner and foreign substances being adhered to the cleaning blade and to prevent the occurrence of noise caused by the straight contact of the cleaning blade with a photosensitive body.
Japanese Laid-Open Patent Application No. 6-051673 (Prior Art 4) discloses a cleaning unit including vibration application means that comes into contact with the surface of a photosensitive body and applies vibration thereto. The vibration application means generates vibration vertically, laterally, or vertically and laterally so as to increase the capability of cleaning residual toner.
Japanese Laid-Open Patent Application No. 11-030938 (Prior Art 5) discloses a cleaning unit including vibration means provided to the fixed end (non-cleaning part) of a cleaning blade, the vibration means applying vibration to the cleaning blade so as to loosen the particles and remove the particles from a surface.
However, the cleaning unit according to Prior Art 1, which includes the cleaning brush pulverizing the residual toner so as to produce pulverized toner on the photosensitive body, is large in size. Further, it is very difficult to pulverize toner formed of resin, and even if it is possible to pulverize the toner, the pulverization causes damage to the surface of the image carrier, thus degrading image quality.
According to the image-forming apparatus of Prior Art 2, which uses the powdery mixture material including the irregularly shaped toner having a smaller average particle size than the spherical toner, the merit of improved image quality obtained by using the spherical toner is reduced, thus resulting in the degradation of image quality. The irregularly shaped toner may be transferred so as to degrade the quality of dots formed by the toner.
According to Prior Art 3, vibration application means causing large displacement is required to produce the desired effect, and the practical application of the technique of Prior Art 3 to high-speed printing is difficult.
According to Prior Art 4, it is difficult to produce the desired effect over the entire width of the photosensitive body. The same applies to Prior Art 5.
Further, the cleaning units of Prior Art 3, 4 and 5 only apply vibration to shake off toner adhering to the cleaning blade or loosen toner from the surface of the image carrier, and do not apply vibration responding to the mechanism of the occurrence of imperfect cleaning of the spherical toner. Accordingly, imperfect cleaning occurs in the case of the spherical toner.
Therefore, the inventors of the present invention have studied the mechanism of the occurrence of imperfect cleaning caused by a counter-type cleaning blade in the case of using the spherical toner, and have clarified the cause of the occurrence of imperfect cleaning.
FIG. 1 is a diagram showing a cleaning unit using a typical counter-type cleaning blade 101. According to the cleaning unit of FIG. 1, the end (free end) of the cleaning blade 101 held by a metal holder 100 is caused to come into contact with an image carrier 111 in the counter direction of a direction indicated by arrow A in which the image carrier 111 rotates so that an angle θ is formed between a ventral surface 101c of the cleaning blade 101 and the surface of the image carrier 111. The free end of the cleaning blade 101 is pressed against the image carrier 111 by an amount d so as to remove residual toner on the image carrier 111.
Normally, the conventional cleaning blade 101 is an elastic rubber member whose principal component is polyurethane rubber. Generally, the cleaning blade 101 has a JIS-A hardness of 65 to 70°, a thickness of approximately 1.5 to 2.0 mm, and a blade free length (protrusion) from the metal holder of 8 to 15 mm, and the contact angle θ is set to 20 to 30°.
When the image carrier 111 rotates with the cleaning blade thus being in contact therewith, the movement of the image carrier 111 in the A direction causes an edge part 101b of a cut surface (end surface) 101a of the cleaning blade 101, which is an elastic member, to be pulled in the A direction by the friction between the edge part 101b and the image carrier 111. As a result, the cut surface 101a of the blade 101 deforms to turn in the A direction as shown in FIG. 2. This turning of the cut surface 101a forms a nip part N having a wedge-like shape between the cut surface 101a of the end of the blade 101 and the image carrier 111.
In this case, if pulverized toner Ta is used, the edge portions of the distortedly shaped particles of the pulverized toner Ta are caught in the wedge-shaped nip part N formed between the cleaning blade 101 and the image carrier 111 as shown in FIG. 3. At this point, repulsion to bring back the deformed portion of the cut surface 101a of the blade 101 to its original state is exerted thereon, thus causing so-called stick and slip motion to occur.
A description is now given, with reference to FIG. 4, of the stick and slip motion. When the blade nip sticks to the surface of the moving image carrier 111, the blade nip is forced to extend in the rotational direction of the image carrier 111 (the A direction) as indicated by a broken line in FIG. 4. When the blade nip is extended to a certain position, the repulsion of the blade 101 becomes so large that the blade nip slides on the surface of the image carrier 111 when a static frictional force and the repulsion are balanced. When the blade nip slides on the image carrier, a coefficient of dynamic friction is smaller than a coefficient of static friction. Accordingly, the blade nip returns to its original position (indicated by a solid line) while slipping on the surface of the image carrier 111. The returning force of the repeated stick and slip motion (whose range is indicated by SP in FIG. 4) causes the toner Ta remaining in the wedge-like nip part N to receive a force to return the toner Ta in the direction opposite to the traveling direction of the image carrier 111. As a result, the toner Ta is cleaned.
On the other hand, a description is given below, with reference to FIG. 5, of the case of employing the spherical toner. FIG. 5 is a diagram showing the behavior of spherical toner Tb when the spherical toner Tb enters the wedge-shaped nip part N formed between the cleaning blade 101 and the image carrier 111.
In the case of using the spherical toner Tb, its particles, which, unlike those of the pulverized toner Ta, have no distorted portions, are not caught by the end part of the blade 101. Therefore, the spherical toner Tb, entering the wedge-shaped nip part N to be held between the blade 101 and the image carrier 111, receives a moment rotating because of the friction between the spherical toner Tb and the image carrier 111 with their contact part serving as a driving source. Accordingly, the spherical toner Tb, rotating in the direction opposite to the traveling direction of the image carrier 111, moves in the same direction as the rotational direction of the image carrier 111 to slip through between the blade 101 and the image carrier 111, thus resulting in imperfect cleaning.
At this point, once the “slip through” of the spherical toner Tb occurs, the spherical toner Tb functions as lubricant between the cleaning blade 101 and the image carrier 111 as shown in FIG. 6. The spherical toner Tb functions to reduce the friction between the end part of the blade 101 and the image carrier 111 and release the turning of the cut surface 101a of the blade 101 (or return the blade 101 to its original shape). Therefore, the above-described stick and slip motion, which is the basic function of the cleaning by the blade 101, is prevented from occurring, thus causing the phenomenon of the successive occurrences of imperfect toner cleaning (removal).
The above description is given of the mechanism of the occurrence of imperfect cleaning of the spherical toner. On the other hand, in the case of toner of a small particle size, it has also been confirmed that the smaller the toner particles are, the more easily the toner enters the wedge-shaped nip part N shown in FIG. 3. Further, it has also been confirmed that even if the particles of the toner that has entered the nip part N are distorted in shape, the toner particles are caught in the nip part N less easily as the toner particles become smaller in size, thus making it easier for the toner particles of a smaller size to slip through between the blade 101 and the image carrier 111.